. Outlines of zoology. a-ture of the tail, and an anterior, giving rise to the blood, connectivetissues, body muscles, excretory and genital organs. According to \an GENERAL NOTES ON TUNIC ATA. 451 Beneden and Julin, the mesoderm primarily originates in the form oftwo pockets, which grow out from the gut, as , andwhose cavity is the true coelom. According to the majority of investi-gators, it originates as solid blocks of cells, and the body cavity isonly represented by spaces produced by the subsequent separation ofthese cells. The further processes of development result in the fo
. Outlines of zoology. a-ture of the tail, and an anterior, giving rise to the blood, connectivetissues, body muscles, excretory and genital organs. According to \an GENERAL NOTES ON TUNIC ATA. 451 Beneden and Julin, the mesoderm primarily originates in the form oftwo pockets, which grow out from the gut, as , andwhose cavity is the true coelom. According to the majority of investi-gators, it originates as solid blocks of cells, and the body cavity isonly represented by spaces produced by the subsequent separation ofthese cells. The further processes of development result in the formation ofa tadpole-like larva, with dorsal nervous system, notochord in thetail region, and well-developed sense organs. Two ectodermal in-vaginations form the original double peribranchial chamber, andsmall- diverticula from the pharynx meet these and form the firstgill-^; For sjme kaurs the larva enjoys a free-swimming life, using its tailas an organ of locomotion. Then it fixes itself by papillae on its Fig. 243.—Embryo of ClaveUna.—MoiifLtA after Seeliger. Fixing papilla; ectodermic fold; c.£^., ciliated groove;en., endostyle; , cerebral vesicle with sense organs; ^j.,gill-slits; »., nerve-cord beginning to degenerate; ch., noto-chord ; g., gut curving upwards towards atrial opening. Theatrial invagination is marked by a dotted line ; the mouth andatrial opening are indicated by arrows. and begins almost immediately to degenerate. The tail shrinks anddisappears, being consumed by phagocytes. The nerve-cord is lost,and with it the larval sense organs, while simultaneously a change ofaxis results in the adult relation of parts. The peribranchial chamberbecomes greatly enlarged, and its two openings fuse together to form thesingle atrial aperture of the adult. The gill-slits increase greatly innumber, the increase being due both to the formation of new slits andto the division of those first formed, and the whole animal under-goes metamorpho
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
